1. Field of the Invention
This invention relates to a thermal transfer recording material for image recording by a thermal transfer system in which recording is carried out by allowing a dye to migrate from a dye donating material to an image receiving material by heating correspondingly to an image information.
2. Description of the Prior Art
A thermal transfer system has lately been developed to obtain a print from an image electronically formed by means of a color video camera. According to the method of obtaining such a print, a color-separated image is converted into an electrical signal which is then transmitted by wireless to a printer. To obtain a print, the dye donating material is face to face superposed on the image receiving material and inserted between a thermal head and platen roller. The thermal head is heated by turning on electric current according to the electrical signal transmitted. The dye donating material is heated from the back surface by the thermal head and the dye in the dye donating material is allowed to migrate to the image receiving material and imagewise recorded. In the case of a color image, this procedure is carried out by using in order yellow, magenta and cyan and optionally, a black dye donating material and heating to record the image.
Another method for thermally obtaining a print using the above described electrical signal is a method comprising using a laser instead of the thermal head. In this method, as a dye donating material, there is used a material strongly absorbing a laser light. When the laser light is irradiated on the dye donating material, this light-absorbable material converts the light energy into therermal energy, heats the dye donating material and thus, the dye is removed to the image receiving material. This light-absorbable material is contained in the dye donating layer or in contact with it. The laser light is modulated by an electrical signal and subjected to heating of the dye donating material.
It is preferred that as the dye donating material in the above described thermal transfer recording system, there is used a dye as readily thermally migratable as possible for the purpose of decreasing the load to the thermal head and increasing the recording rate. However, such a problem thus arises that the dye is precipitated or the dye is removed in the recorded image receiving material during storage or when allowed to stand in an atmosphere at a high temperture and high humidity, thus lowering the sharpness of an image or migrating to a substance in contact with it and contaminating the substance.
In order to solve this problem, there have been proposed methods comprising incorporating a mordant in an image receiving layer and thermally transferring a dye having a mordanting group such as phenolic hydroxyl group, etc., as disclosed in Japanese Patent Publication No. 15760/1992 and Japanese Patent Laid-Open Publication Nos. 188391/1989 and 83685/1991, and it has been proposed to use an aryl azo dye of phenol-type, as disclosed in Japanese Patent Laid-Open Publication No. 219061/1994.
According to these methods, the transferred dye is mordanted and is hard to be removed in the image receiving material, so the problem can be solved that the sharpness of an image is lowered and the dye migrates to a substance in contact with it to contaminate the substance.
The heat-migratable dyes used in these systems have various limitations and very few ones satisfy all the properties required. The properties required are, for example, that (1) the dye is readily heat-migrated, (2) the dye dissociates to give anion with a basic material and/or mordant contained in an image receiving layer and show spectral property preferable for color reproduction, (3) the dye is resistant to light or heat and resistant to various chemicals, (4) an image does not fade and the sharpness is not lowered, (5) retransferring of an image does not occur and (6) a heat-transfer dye donating material can readily be prepared.